In a computer network handling sensitive data or physical processes, portions of the network may be connected by one-way links. The term “one-way link” is used in the context of the present patent application and in the claims to refer to a communication link that is physically configured to carry signals in one direction and to be incapable of carrying signals in the opposite direction. One-way links may be implemented, for example, using Waterfall® systems, which are manufactured by Waterfall Security Solutions, Ltd. (Rosh HaAyin, Israel). The Waterfall system provides a physical one-way connection based on fiberoptic communication, using an underlying proprietary transfer protocol. When a transmitting computer is connected by a Waterfall system (or other one-way link) to a receiving computer, the receiving computer can receive data from the transmitting computer but has no physical means of sending any return communications to the transmitting computer.
One-way links may be used to prevent data either from entering or leaving a protected facility. For example, confidential data that must not be accessed from external sites may be stored on a computer that is configured to receive data over a one-way link and has no physical outgoing link over which data might be transmitted to an external site. On the other hand, in some applications, the operator of the protected facility may be prepared to allow data to exit the facility freely via a one-way link, while preventing data from entering the facility in order to thwart hackers and cyber-terrorists. Unlike conventional firewalls, one-way links permit information to leave a protected facility without risk to the safety or availability of the network in the facility due to attacks originating on an external network. In practice, however, there is sometimes a need to transmit at least small amounts of information from an external network back into the protected facility.
U.S. Patent Application Publication 2014/0068712, whose disclosure is incorporated herein by reference, describes apparatus and methods for automatically controlling inputs to a protected destination. In a disclosed embodiment, communication apparatus includes a one-way, hardware-actuated data relay, which includes a first hardware interface configured to receive a command from a communications network and a second hardware interface configured to convey the received command to a protected destination when the relay is actuated. A decoder includes a third hardware interface configured to receive a digital signature for the command from the communications network and hardware decoding logic coupled to verify the digital signature and to actuate the relay upon verifying the digital signature, whereby the command is conveyed via the second hardware interface to the protected destination.
U.S. Patent Application Publication 2016/0112384, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a method for communication that includes receiving in a secure installation via a network from a remote user terminal an input comprising a stream of symbols that has been encrypted using a preselected encryption key. The encrypted stream of symbols is decoded in the secure installation using a decryption key corresponding to the preselected encryption key, to produce a clear stream of symbols. A computer program running on a processor in the secure installation is used in processing the symbols in the clear stream and generating a graphical output in a predefined display format in response to processing the symbols. The graphical output is outputted from the secure installation to the network in an unencrypted format for display on the remote user terminal.